An important performance feature of a television receiver system is an ability to display a picture almost immediately after a system perturbation such as a channel change. In the case of a television receiver including digital signal processing circuits, the first of many events that must occur to produce a picture display is the development of an appropriate clock timing signal for use by the digital signal processing circuits.
One particularly advantageous digital signal transmitting/receiving system for a high definition television (HDTV) signal is described in my copending U.S. patent application Ser. No. 650,329 titled "Modulator/Demodulator for Compatible High Definition Television System", filed Feb. 4, 1991. In that system, transmitted high definition television information is divided into high priority information that is intended to be received with high reliability, and low priority information. The high priority information and the low priority information are conveyed as separate QAM (quadrature amplitude modulated) carrier signals within different portions of the television signal frequency spectrum. The high priority information exhibits a small excess bandwidth, and a narrower bandwidth and significantly larger amplitude than the low priority information.
At a receiver, a clock signal for use by associated digital signal processing networks is developed from the high priority narrowband signal by means of a nonlinear signal generator and a phase locked loop responsive to the output signal from the nonlinear signal generator. Since the nonlinear signal generator operates on a narrowband QAM signal with small excess bandwidth, the magnitude of the output signal from the nonlinear generator is small. This dictates the use of a narrowband phase locked loop, which exhibits an undesirably slow response time. Signal energy to assist in developing the clock signal can be added to the television signal in the form of a pilot signal, but the energy associated with such an added signal may degrade the television signal by introducing interference and unwanted artifacts. In any case, in a system of this type it is desirable to provide a clock signal almost immediately after a channel change because viewers typically prefer seeing a displayed image for the new channel almost immediately after a channel change. The present invention addresses this matter.